1. Field of the Invention
This invention relates to a coiler-furnace unit comprising a coiler having a mandrel for carrying the strip which is to be heated.
2. Description of the Prior Art
For a compensation of heat losses suffered by a strip during hot rolling, it is usual to provide coiler-furnace units before and behind reversible hot-rolling mill stands. In such coiler-furnace units the strip to be rolled is heated between successive passes. The coiler-furnace unit usually comprises a heat-shielding hood and a coiler mandrel protruding into said the hood. The strip wound on the mandrel is heated by hot flue gases which promote the formation of undesirable scale. The scale is wound in between the convolutions of the coiled strip and is subsequently rolled into the strip so that the surface finish of the resulting hot-rolled strip may be highly adversely affected. Besides, the sulfur contained in the flue gases deteriorates the nickel-containing steel of which such coiler mandrels are usually made.
In order to avoid these disadvantages, it has been proposed in French Pat. No. 1,427,524 to heat the strip electrically by means of a resistance heater in which the strip is used as a heating resistor. For this purpose, electric current is conducted through the strip between the coiler and pinch rollers preceding the coiler. But because no heat insulation is provided in this arrangement, it has not been able to heat the strip to sufficiently high rolling temperatures with an economical expenditure of energy. Besides, scale may be formed by sparks produced where the current enters the strip. Finally, a considerable cropping loss is involved because the strip is heated between the coiler and the pinch rollers preceding the coiler so that the strip cannot be entirely wound on and unwound from the coiler.